Numerous drop on demand printing devices, which deliver ink droplets to paper or other material to be marked, are well known. Such mechanisms include, for example, remote solenoid valves feeding nozzles through a length of tubing, solenoid valves operating at the nozzle, and solenoids operating via flexible or rigid cable. See, for example, U.S. Pat. Nos. 4,723,131, 4,576,111 and 4,809,017. These systems produce large drops suitable for large character printing such as that employed on cardboard cartons.
Another example of a prior art valve device is shown in PCT International Publication No. WO 85/01103, published Mar. 14, 1985. In that device, a coil driven magnetic reed operates as a valve for opening an closing an ink chamber, thereby dispensing drops of ink upon actuation of the valve.
Devices such as those described suffer from a number of problems. An important problem related to remote solenoid devices is the low frequency response of the valve/nozzle assembly due to slow valve action, causing long filaments of ink to be ejected from the nozzle. Other problems found in earlier systems include the ingestion of air into the nozzle, causing printing errors, and high power requirements for actuating the solenoid driven valve drivers.
Proximate valve assemblies, and assemblies employing cables between a solenoid and the ink delivery nozzle have encountered reliability problems. In addition, the high manufacturing costs associated with such devices, and low frequency response problems like that described above, have limited the acceptance of such devices.
Another type of valve assembly is used in small character printers (such as document printers). Such devices use the surface tension at the orifice of the nozzle to provide the function of a valve. This surface tension absorbs the recoil of the ink drop to stop leakage from the orifice. A small orifice (typically less than 60 microns) must be used, and the surface tension of the ink must be high, to prevent leakage. Such systems have not proved suitable for large character printers which require larger ink drops.
Another problem found in many of the previously described systems is contact between the ink and the valve actuating device. In systems where such contact is permitted, the composition of the ink is required to be non-corrosive to limit the damage caused by such contact.